In a satellite communication system, information is exchanged between an orbiting satellite and one or more terrestrial-based subscribers. As the satellite moves over the terrestrial-based subscribers, it illuminates a part of the earth's surface using at least one directional antenna beam. The angular energy distribution pattern generated by the satellite antenna includes a main beam through which communication with the terrestrial-based subscribers is achieved. The pattern also includes a sidelobe region of reduced gain response which exists to the sides of the main beam. The main beam of the directional antenna pattern is referred to as the communication beam. The communication beam facilitates the bidirectional information exchange between the satellite and the terrestrial-based subscribers. This information can represent analog or digitized voice, facsimile data, digitized audio or video, internet data, or other type of subscriber information.
It is generally the case that the geographical distribution of terrestrial-based subscribers is not uniform across the surface of the earth. Subscribers tend to be concentrated within specific areas, with these areas separated by great distances. It is often true that the vast majority of terrestrial-based subscribers are concentrated within major metropolitan areas, along major roads and other thoroughfares, and other tightly bounded regions. Given this non-uniformity of subscriber density, it is advantageous to illuminate only those regions of the earth's surface which contain subscribers.
In a present day satellite communication system, such as the IRIDIUM.RTM. system, each communication beam subtends a fixed angular area. Each beam can be shared by multiple subscribers through the use of a unique frequency (in an FDMA system), or time slot (in a TDMA system), or may use another method of uniquely identifying the subscriber such as employing a unique spreading code in a CDMA system. Each of the unique divisions within a communication beam is referred to as a satellite channel. The satellite radiates a unique channel for each terrestrial-based subscriber of the satellite system when the subscriber is engaged in a call. In a system such as IRIDIUM.RTM., a beam can cover an area hundreds of miles wide.
Communication beams are generated by the satellite in response to as few as a single subscriber requiring connection with the satellite. It is generally desirable to provide as few communication beams as possible in order to minimize the resources required by the satellite to conduct communications with the terrestrial based subscribers. However, given that subscribers may be distributed over a wide geographical area, many beams must be generated by the satellite in order to provide connectivity with each of the terrestrial-based subscribers. This results in some of the energy from the satellite being radiated onto areas which do not contain subscribers. This increases the demand on the satellite resources needed to generate and maintain the communication beams.
An additional drawback to the use of communication beams which subtend a fixed angular area, is the relative difficulty for the satellite system to selectively avoid interference from terrestrial-based interfering stations. Since the communication beam generally possesses a defined pattern which includes a fixed main beam gain value as well as defined sidelobe levels, it is often difficult to overlay the maximum gain portion of the beam on an area which contains subscribers while simultaneously placing a minimum gain portion of the beam, such as a sidelobe, on an area which contains an interfering station. This can have the effect of reducing the quality of service provided to the terrestrial-based subscribers.
Therefore, what are needed are a method and system for overlaying high gain portions of a communication beam on specific geographical areas while overlaying minimum gain portions of a communication beam on other geographical areas. Such a system would provide increased efficiency in the use of satellite resources and provide a higher quality of service to those subscribers proximate with interfering stations.